Multi-ply wood product

ABSTRACT

The invention relates to a multi-ply wood product. The supply of wood suited e.g. for the production of plywood has considerably decreased. Consequently, attempts have been made to compensate the deterioration in the quality of the surface finish and in other wood qualities e.g. by means of various coating methods, with varying success. The most important problem has been the insufficient adhesion between the coating and the wood. For the elimination of these problems, a multi-ply wood product has been developed in which at least one layer is formed by a crude fiber ply blank comprising haphazardly oriented fiber material as well as bonding agents, additives and filler materials. A single hot-pressing step in connection with the production of the multi-ply wood product forms this blank into homogeneous ply (14) which, by virtue of the partial splitting of the fiber structure and the resulting reorientation, which take place during the hot-pressing, fills the irregularities in the surface of an adjacent wood layer (1) and forms a tight synthetic ply of accurate dimensions.

The present invention relates to a multi-ply wood product. Theexpression "multi-ply wood product" is here mainly meant to includeplywood and laminated veneer lumber.

Plywood industries all over the world have in recent years beencompelled to face the fact that it is becoming increasingly difficult toobtain wood of an appropriate quality. The supply of birch wood for woodplies has decreased to such an extent that it is today difficult tomaintain the traditionally high quality of plywood; birch wood is verywell suited for the production of plies as it is compact and possesses ahigh wear resistance. Due to the decreased supply it has becomenecessary to replace birch with other kinds of wood, e.g. softwood, toan increasing degree. Wood plies made of softwood cause problems bothfor the manufacturer and the user of plywood. Softwood plies have acoarser surface than birch plies. They fracture more easily, and thegrossgrained pieces of wood at knot points do not properly stay inplace. The traditional production processes of plywood can utilize onlyabout 25 per cent of the obtained raw material. The rest requires newspecial measures in order to maintain a reasonable quality.

Deterioration in the quality of plywood has caused reactions everywherein the world. Plywood industries in e.g. Sweden, the Soviet Union andFrance have made earnest appeals for improvement in this matter. Forexample, it has been found out that plywood is today more liable tofracture than previously. Neither is it suited for various purposes aswell as previously. Plywood causes problems e.g. in the molds ofconcrete casting. It does not withstand the strains caused by theconcrete flow or the vibration of concrete but is broken so thatfreezing water gets into the cracks, and fractures are formed. Nor doesit absorb water and, consequently, water remains between the concreteand the mold, and the formed air bubbles leave marks in the hardenedconcrete. It has also been found that the impact strength of plywood islower than previously.

Various efforts have been made to solve the problems. Superficialdefects, such as recesses and cracks, affecting the appearance of theplywood, have been evened with a filler. This may be acceptable if thedefects are small and not too numerous. As compared with the surroundingwood surface, a defect nevertheless forms an area of discontinuedhomogeneity which may be the root of problems, depending on the use. Thefiller does not eliminate the fracture effect caused by the defect;furthermore, the filler often comes off partially or wholly so thatmoisture gets in between the plies, and a stripping process is started.Defects caused by knots have been repaired by means of a circular pieceof homogeneous woodply which is fitted accurately in a hole made roundthe knot and fastened in the woodply beneath in the single pressing ofthe plywood sheet. After the pressing, the sheet is ground. With respectto the fracture effect, this way of repairing is better, because thedefect is confined within smoothly curving contours. However, thecircular piece does not always stay in place. When the plies are apartfrom each other before the pressing step, the piece is glued in the plyonly at the edges thereof. When handling the ply, the piece is oftendisplaced or comes off completely. A further problem arises in that anunnecessarily accurate control is required to provide the circular piecewith similar shrinking and swelling properties as the surrounding woodply. It often happens that the circular piece is curved or cracked dueto the greater swelling thereof, or comes off at the edges thereof dueto greater shrinking. In general, all the repairs are carried outmanually. In view of the manufacturing costs, this is a drawback. Therepairs do not notably improve the wear resistance of the plywood, noris its resistance to weather improved at the repaired points. Theappearance, either, is not improved to any greater degree by the localrepairings. Thus, it is obvious that this kind of problems are furtheraggravated as the quality of wood used for plies is deterioratingeverywhere.

As to the plywood products in which the preservation of the natural woodpattern of the outer surface of the plywood is of primary importance,the outer surface of the plywood is covered with a transparent membranewhich is bonded to the wood surface by means of a resin by hot-pressing.The membrane is a thin film manufactured in advance by pressing, and itis pressed in place in connection with the single pressing of theplywood plate. It covers the defects occuring in the wood surface atleast to such an extent that they are not too easily noticeable,including possible local repairings. The wood surface may then be rathercoarse and of inferior quality. U.S. Pat. No. 4,084,996 deals with thiskind of prior art, and it teaches that a thin cellulose filmmanufactured in advance is hot-pressed on the outer surface of theplywood by single pressing, with phenol formaldehyde resin as a bondingagent. It can be said that the publication represents the knowntechnique for the manufacture of coated plywood sheets at present on themarket, irrespective of whether the membrane is transparent or not. Anexample of this kind of plywood sheets would be e.g. plywood sheetscoated with a so called painting base paper or a so called concretingmembrane. Essential is that the fiber structure of the membrane used inthe production gets its final shape at an earlier production stage.Consequently, the fibers of the membrane are not any longer able tofollow the contours of the defects of the wood surface in the fixingpressing.

It is also known to coat the wood surface in one way or another e.g.with a layer of glass fiber which is bonded by means of a resin, such asa polyester. The coating is usually constructed manually layer by layer.The wood surface may be rather coarse and of inferior quality. Thecoating adheres properly to the surface of the wood if the surface isclean, and it gives a good protection against any disadvantageous outereffects. Also, it considerably improves the strength of the structure.However, the method is too slow to be applied to mechanical productionof plywood sheets, in which the desired final qualities have to beachieved by a single pressing.

A common drawback of known coating methods is that the adhesion betweenthe coating and the surface of the wood is not sufficient for allapplications. In such cases as disclosed in the above-mentionedpublication, this is due to the unsufficient moldability of themembrane. Although it is elastic and resilient, it does not stretch intoevery corner of the recesses. Due to the preforming of the membrane, thefibers are so oriented in the direction of the plane of the membranethat they are bent over the sharp corners of the recesses when pressedagainst the wood surface, so that an empty space remains under themembrane. Besides macroscopic recesses, there are numerous sharpmicroscopic recesses on the surface of the wood, especially in pineplies. The dense membrane structure does not reach into these recesses.As a result, the thin intermediate layer between the coating and thesurface of the wood is not homogeneous. The shearing stresses arethereby in some places transmitted over areas which are not reinforcedwith fibers. This, of course, affects adversely the strength of thecoated plywood sheet as a whole, as compared with a plywood sheet havinga homogeneous adhesion layer. A coating which is not pressed on to thesurface of the wood is even more greatly affected by this adhesiondrawback.

The object of the present invention is to obviate the above drawback s.

Thus, the invention is concerned with a multi-ply wood product which ismainly characterized by what is disclosed in the claim 1. Claim 1discloses a multi-ply wood product, characterized in that at least onelayer thereof is formed by an integral ply formed in connection with theproduction of the multi-ply wood product by hot-pressing, in a singlestep, of a crude fiber ply blank comprising haphazardly oriented fibermaterial, a bonding agent, additives, and filler material, which plyblank, by virtue of the haphazard orientation of its fibers, fills theirregularities of the surface of an adjacent wood layer during thehot-pressing step, and thus forms a compact synthetic ply of accuratedimensions. Preferred embodiments are described in more detail in theclaims 2 to 10.

According to the invention, the multi-ply wood product is provided witha synthetic ply which is bonded to a wood ply or wood plies by means ofa resin by a single step of hot-pressing in the production of themulti-ply wood product. As distinct from the prior art, the syntheticply, according to the invention, changes in structure when being pressedin to final position. The synthetic ply initially has a low density andis relatively thick. In this stage, the ply is here called a ply blank.In other words, this ply blank is an elastic fiber composite, light instructure and comprising at least substantially unpressed fibermaterial, a bonding agent and possibly additives and/or fillermaterials. The blank is manufactured in advance and is either positionedas such to form the outer surface of the multi-ply wood product, or ispositioned between wood plies before the pressing of the product. At thepressing stage of the multi-ply wood product, the ply blank is pressedinto a synthetic ply in a single hot-pressing step so that it in thefinished wood product forms a structural part the thickness and functionof which is similar to those of the wood plies. Since the fibers of theply blank are haphazardly oriented, they reach into every corner of therecesses. The coating is homogeneous and, consequently, the strength ofthe entire multi-ply wood product is higher than that of a structurecoated with a thin preformed fiber film. Due to the homogeneity, thefracture initiates are considerably less numerous in spite of the factthat the number of the wood ply surface defects may be much greater,i.e. the quality of the wood ply may be much lower than in known coatingapplications. By virtue of the better adhesion, the coating is alsosuperior in that the wood product is less exposed to the effects ofweather. The resistance of the surface is increased by the merethickness of the synthetic ply as it has roughly the same thickness asthe wood plies. But even if the synthetic ply would fracture due tomechanical strains, possible water damage is decisively slowed down byvirtue of the high homogeneity and adhesion in the area around thefracture point.

The ply blank is treated according to the requirements of intendedpurposes. Its basic structure comprises a combination of a fibermaterial, preferably mineral wool (a glass fiber, mineral fiber, slagfiber, or ceramic fiber) and a resin. In special applications, themineral wool can, if required, be replaced with a cellulose fiber orsome other organic fiber or, more generally, with a fiber which can beopened by machining so that fibers adhering to each other form a firmnet. The bonding agent is preferably a resin of high fire-resistance. Inspecial applications, the ply blank is treated according to therequirements in each particular case. Such a special requirement may bee.g. an improved fire-resistance, a required hygroscopicity, a requiredsuppression of reverberation, a low natural vibration spectrum, etc. Theprimary object, however, is to provide an even, integral, continuoussynthetic ply which at least maintains the traditionally good qualitiesof plywood even though the quality of wood plies would deteriorateconsiderably. This object is achieved by means of a ply blank having thebasic structure comprising at least mineral wool and resin.

The aforementioned aspects relating to plywood essentially apply forlaminated veneer lumber as well. In principle, laminated veneer lumberdiffers from plywood in being thicker, i.e. having more plies, up toabout 60 to 160 millimeters, and in that the grain direction of the woodplies are at least essentially in parallel, lengthwise. Laminated veneerlumber is used e.g. for girders in building construction. The U.S. Pat.No. 3,908,725 represents the prior art in this field.

The Finnish Patent Specification 842,695 discloses a composite membranewhich is composed of broken mineral fibers bonded to each other with apolymerized bonding agent and which is intended e.g. for buildingplates. This composite membrane and the fibers thereof, however, arehot-pressed in advance, and the fibers are bonded by means of resin. Theuse of the composite membrane requires two separate hot-pressing steps:one when the membrane is manufactured and the other when it is fixed inplace.

In the following the invention will be described in more detail withreference to the attached schematical drawing.

FIG. 1 is an exaggerated cross-sectional view of a plywood sheetaccording to the prior art.

FIG. 2 is a cross-sectional view of a multi-ply wood product (plywood)according to the invention.

FIG. 3 illustrates a preferred embodiment of laminated veneer lumber.

In FIG. 1, the reference numerals 1, 2 and 3 designate three wood plieswhich are glued one upon another so as to form a plywood plate. Defectsin the wood plies, such as recesses 4 and 6, are covered with a membrane5 bonded with a resin. Due to the insufficient resilience of themembrane, it does not follow the contours of the recesses but bends overthem, as e.g. at a point 7. The membrane may also swell under theinfluence of the pressure created by steam or gases generated during thehotpressing, as e.g. at a point 8. Thus, the coating is not homogeneous.The fragile coating may fracture at a cavity, and the recesses may stillbring about a further fracture effect. The surface of intermediate pliesmay be repaired as described above e.g. by means of a circular piece 9positioned at a knot 10, or by means of a filler 11; however, the resultis hardly satisfactory. In the embodiment according to FIG. 2, i.e. thataccording to the invention, the fibers of coating 14 reach into to thebottom of the recesses. If an unusually high strength is required, oneor more wood plies are replaced with a synthetic ply. The synthetic ply15 of FIG. 2 generally illustrates such a replacement.

In principle, the mineral wool of the synthetic ply is qualitativelysimilar to the wool obtained immediately after the wool chamber of aconventional mineral wool production line. Thus, it has a low density.The density of the ply blank preferably varies from 50 to 200 kg/m³ Ifheavy filler material are used, the density may be higher. The ply blankmay also contain an original bonding agent, but this is not necessary.If required, the ply blank is pressed lightly into an advantageousinitial thickness. However, it is important that the initial length ofthe fibers, i.e. ≦100 mm, with mineral wool preferably ≦10 mm, remainsunchanged as well as the haphazard orientation of the fibers. At thefinal stage, i.e. when the ply blank is pressed to form a synthetic plyof a multiply wood product, its density is 100 to 5,000 kg/m³, and thelength of the fibers ≦10 mm, for mineral wool preferably ≦300 μm. Socalled technical wool may well be used, such wool is not especiallyintended for heat insulation applications. The expression "crude fiberply blank" here refers to the stage before the hot-pressing step.

A preferred bonding agent for use in ply blanks here contemplated issuch, that it is activated at the required pressing temperature, i.e.within the range from 50 to 350° C. The bonding agent is preferablypresent in the amount from 5 to 70 per cent by weight of the fibermaterial. If the application requires the use of a bonding agent whichrequires an unusually great amount of heat to be transmitted through thestructure to be pressed, the temperature is maintained at an essentiallyconstant value within said range while the pressing time is made longer.If no special requirements exist, a chlorine-containing polymer, suchas, a PVC-type or a generally chlorine containing resin, is used as abonding agent for improving the fire-resistance. In general,thermoplastic resins are advantageous, because they melt at a moderatelyraised temperature and are restored to their initial state when theycool. Thermosetting resins lose their bonding properties if thetemperature raises too much. However, an advantage thereof is that theyresist higher temperatures than thermoplastic resins. Having a greaterelasticity, they are more suitable for the transmission of the shearingforces generated in sandwich structures. In addition, they have a higherresistance to tension. A compromise has to be resorted to in the choiceof resin; i.e. such a resin is chosen which meets the requirements ofthe primary function of the synthetic ply in the best possible way.

If a special application requires the choice of an organic fiber or,more generally, a fiber which can be opened by machining, a preferredopening degree is 10 to 30° SR. Such fibers are e.g. cellulose fibers,animal furs, asbestiform fibers, etc.; more generally, fibers which canbe made to adhere to each other by machining so that a firm fiber net isformed.

The above-mentioned hygroscopicity and the suppression of reverberationcan be adjusted by an advantageous porosity of the synthetic ply. Theporosity here means that portion of the volume of the synthetic plywhich does not contain solid substances. Preferably, the porositydesired lies in the range from 0 to 80 percent per volume. The desiredporosity can be obtained e.g. by the use of a polycondensation resinsuch as phenol formaldehyde resin as a bonding agent. The naturalvibration spectrum of the multi-ply product can be shifted to lowerfrequencies by adding an advantageous amount of a heavy filler material,such as a mineral powder or components based on barium, tungsten and/orlead, to the ply blank.

An improved fire-resistance is achieved by adding to the ply blank e.g.antimony trioxide or dihydrate gypsym.

The above shows only limitedly how a ply blank provides a multi-ply woodproduct with desired properties. All possibilities cannot, of course, bedescribed in this connection. A primary object is to make plywood sheetsmade of wood plies of low quality such, that they meet the expectedrequirements. This is achieved according to the invention in such amanner that an advantageous number of the wood plies of the multi-plywood product is replaced with synthetic plies comprising a fiber/bondingagent composite as well as additives and filler materials possibly addedthereto. The synthetic ply is positioned, in its initial form, as aready blank, between the wood plies to be pressed into a sheet andobtains its final form as a synthetic ply of the multi-ply product inthe single hot-pressing step for forming the multi-ply wood product. Itis thereby self-evident that the invention also relates to all thosenumerous special properties which are required from a plywood productand which can be achieved by means of the preferred composition of theply blank.

In the following a few examples are set forth to illustrate thepossibilities offered by the method according to the invention. Thestrength of the sheets is used as a criterion in the comparison, becausethis is the most important criterion of the usability of plywood.

EXAMPLE 1

A strength test was carried out in which the strength values ofgeometrically identical wood plies and pieces of synthetic pliesmanufactured according to the invention and by means of conventionalpressing pressures were compared separately under identical conditionsand under identical stress.

The fiber material of the synthetic ply was glass fiber having thedensity of 1,500 kg/m³, and a phenol formaldehyde resin having aconcentration of 30 per cent by weight was used as a bonding agent. Theporosity of the synthetic ply was 25 per cent per volume. The followingresults were obtained:

    ______________________________________                                        Ultimate stress                                                                            Wood ply         Synthetic                                       N/mm.sup.2   Birch   Pine     Spruce                                                                              ply                                       ______________________________________                                        Tension      11,5    10,6     11,0   70                                       Pressure      5,9     6,4      3,5  160                                       Bending      13,6    11,8      8,3  160                                       ______________________________________                                    

EXAMPLE 2

Three birch plies having the thickness of 1.3 mm were treated accordingto the conventional plywood production practices, and were positionedone upon another into a hot press. A synthetic ply blank manufacturedaccording to the invention and having the thickness of about 15 mm andthe density of 100 kg/m³ was positioned on the uppermost wood ply andunder the lowest wood ply. The ply blank comprised mineral wool andphenol formaldehyde resin having the concentration of 35 per cent byweight calculated on the weight of the blank. The whole package waspressed during 4 minutes at a temperature of 130° C. and a pressure of1.6 MPa.

After the pressing the thickness of the synthetic plies was 1 mm, thedensity 1,400 kg/m³, and the porosity 25 per cent per volume, and thethickness of the entire coated plywood plate was 6 mm.

For the strength comparison, another similar sheet was made of the sameplies, the outer surfaces of which sheet were coated only with a thinlayer of resin; and still another, conventional plywood plate, i.e. aplate the outer surface of which were not treated. The plies were chosenaccurately so that the patterns of the defects in the plies of the testplates were as identical as possible both with respect to the positionand the geometry thereof. All three test plates were subjected toidentical bending tests, using a number of strips cut off the plates inan identical manner. The following results were obtained:

    ______________________________________                                                               Bending stress                                         Test sheet             N/mm.sup.2                                             ______________________________________                                        Plywood plate + synthetic surface ply                                                                36.3 to 48.7                                           Plywood plate + mere resin                                                                           10.8 to 11.2                                           Mere plywood plate     9.2 to 9.8                                             ______________________________________                                    

As appears from the foregoing, the most essential part of the multi-plyproduct according to the invention is the synthetic ply and its initialform, a ply blank. It mainly comprises mineral wool which can be glasswool, mineral wool or slag wool, and the multi-ply product is preferablyformed by means of a single pressing.

The single pressing has four distinct advantages. Firstly, the singlepressing, of course, reduces costs because there is no other pressing.Secondly, the single pressing is advantageous in that bonds of athermosetting type contained in the wood structure are strained onlyonce. Thirdly, the pressure exerted on the wood structure in theproduction of plywood may approach the break limit of the wood grains.Particularly in the conventional method of two pressings, the woodmaterial is weakened. The method according to the inventionsubstantially reduces this disadvantage. Fourthly, an adhesive layer ofphenol formaldehyde resin between the wood plies is not at all strainedwhen it is in the polymerized state, because it is formed during thesingle pressing.

The properties of a mineral wool coating is superior to that of otherfibers; no delamination deteriorating the properties occurs under thecoating. The coating prevents substances possibly emanating from thewood plies from forming bubbles and breaches on the surface.Traditionally, phenol formaldehyde resins are known for being capable ofadhering both on wood and mineral fibers, which property can be fullyutilized.

By the use of a suitable pressing technique, a plate of accuratedimensions can be provided by means of the mineral wool coating, whichis not possible by means of the prior technique for the manufacture ofplywood because of the nonexistent deformability of the layers to belaminated.

The properties of mineral fiber plies are superior to those of woodplies; in other words, the ply blank strengthens the wood ply structureso that the traditional strength properties can be obtained with a woodmaterial of lower quality. As the mineral ply blank is able to followthe irregularities of the underlying surface and nevertheless form ahomogeneous coating, it is possible to use a ply material of a lowerquality than previously, and at the same time avoid the manual repairingof plies, the surface plies in particular. The ply material according tothe invention has always a higher strength than a knotty and/or repairedwood ply.

The properties of the synthetic ply blank can be varied to a greatextent by changing the components of the ply blank and the proportionsof the components. This can be effected similarly as e.g. in the case ofplastic composites reinforced with broken fibers and alloyed plasticcomposites.

In the following a few special advantages are described which can beobtained by means of the synthetic ply blank with a suitablecombinations of the raw materials and which substantially widen the useof plates coated with the coating according to the invention. The basicstructure of the synthetic ply is mainly formed of mineral wool, whichhas the above-described splitting and reorienting properties during thepressing of the final laminate layer.

The structure and the covering properties of the laminate layer can beaffected by varying the different pressing parameters during thepressing (pressing velocity, cycling).

The special properties of the laminate layer (sound, heat) can beaffected substantially by varying the porosity of the surface. A porouslaminate layer can be obtained by a suitable polycondensation resin orby means of different kinds of propellants, or by using insufficientamounts of the bonding agent. Phenol formaldehyde resin is a suitablepolycondensation resin.

The coatability of the laminate layer can be affected e.g. by theporosity, because a porous surface is easier to coat than a completelytight surface.

The fire resistance of the laminate layer, which layer by virtue of thelarge amount of organic material contained therein reduces the fireloadof the wood plate, can be affected by

reducing the amount of the bonding agent,

using less flammable bonding agents,

using fire prevention chemicals,

using inorganic bonding agents,

adding inorganic filler materials to the bonding agent,

adding water and foam forming additives to the bonding agent.

Examples of heat binding substances are hydrous salts or compoundscontaining chemically bound water, such as aluminum hydroxide.

The electric, magnetic and thermal properties of the laminate can bemodified by means of metal and coal fibers.

The use of traditional textile fiber based coatings becomes even moreadvantageous because the adhesion to the surface according to theinvention is extremely good.

The coating can be coloured throughout.

A plate coated according to the invention has stable dimensions and ishomogeneous in all directions.

The manufacture of different kinds of profile pieces of the coatingaccording to the invention is fully possible as well as the profiling ofthe coating e.g. for improving the friction.

By means of the coating according to the invention a surfaceconsiderably harder and more wear-resistant than previously can beobtained.

The coating is completely inert with respect to moisture.

In principle, the multi-ply wood product according to the inventioncompetes with all kinds of coated plywood products which are used today.If the preservation of the wood surface is required, it would beadvisable to coat a synthetic ply blank with a wood ply. The price of amulti-ply wood product coated with the synthetic ply according to theinvention is competitive as compared with prior plywoods, and besidesthe traditional applications the product may even find new applications.

This material coated with synthetic ply has at least the followingpractical applications:

motor and transport industries

surface plates of building industries

casting mold materials

boat building and ship building (intermediate walls)

prefabrication industries

farm building

packing materials.

FIG. 3 illustrates an embodiment of the invention, when applied tolaminated veneer lumber. Reference numeral 20 designates wood plies, 21designates synthetic plies according to the invention, 22 indicates anopening which according to prior art would require a separate fillingpiece, 23 indicates a notch in the wood ply, 24 and 25 indicate jointswhich are displaced lengthwise in order to improve the bend strength ofthe laminated veneer lumber. The joints of the wood plies 20 and thesynthetic plies 21 may be bevelled, as indicated by reference numerals26 and 27.

We claim:
 1. A hot-pressed multi-ply wood product, having at least onewooden ply, comprising at least one layer formed by a prefabricatedcrude fiber ply blank including fiber haphazardly oriented in threedimensions, a bonding agent, additives, and filler material, said plyblank, by virtue of partial breakdown and reorientation of said fiber,fills irregularities of the surface of an adjacent wood ply duringhot-pressing, said hotpressing being carried out in a single step, sothat said layer becomes fully bonded to said adjacent ply, wherein saidfiber is mineral wool.
 2. A multi-ply wood product according to claim 1,wherein the initial density of the crude fiber ply blank is 50 to 2,500kg/m³, and the final density 100 to 5,000 kg/m³, and the initial lengthof the fibers is ≦100 mm, and the final length about ≦10 mm, thediameter of the fibers being 0.5 to 50 um.
 3. A multi-ply wood productaccording to claim 1, wherein the fiber material comprises fibersmachinable at least to an opening degree of 10 to 30° SR.
 4. A multi-plywood product according to claim 1, wherein the amount of the bondingagent is 5 to 70 per cent by weight of the fiber material, and that thebonding agent is activated at a temperature of 50 to 350° C.
 5. Amulti-ply wood product according to claim 1, wherein said prefabricatedcrude fiber ply blank is fitted between a pair of wood plies, so thatafter hot-pressing an intermediate layer of said multi-ply wood productis formed.
 6. A multi-ply wood product according to claim 1, intendedparticularly for applications requiring hygroscopicity or suppression ofacoustic reverberation, wherein the porosity of said layer is 0 to 80percent per volume.
 7. A multi-ply wood product according to claim 1,intended particularly for applications requiring fire-resistance,wherein a chlorine-containing polymer is used as a bonding agent.
 8. Amulti-ply wood product according to claim 1, wherein only one layerthereof is a layer made of wood.
 9. A multi-ply wood product accordingto claim 1, in the form of laminated veneer lumber, wherein every secondply is a compact synthetic ply and every other ply is a wood ply.
 10. Amulti-ply wood product according to claim 1, wherein said mineral woolis selected from a group consisting of glass fiber, mineral fiber, andslag fiber.
 11. A multi-ply wood product according to claim 1, whereinthe initial density of the crude fiber ply blank is 50 to 2,500 kg/m³,and the final density 100 to 5,000 kg/m³, and the initial length of themineral wool fibers is ≦10 mm, and the final length of the mineral woolfibers is ≦1,000 μm, the diameter of the fibers being 0.5 to 50 μm. 12.A multi-ply wood product according to claim 6 wherein said porosity isobtained by the use of a polycondensation resin as a bonding agent. 13.A multi-ply wood product according to claim 6 wherein said porosity isobtained by the use of a thermoplastic material as a bonding agent. 14.A multi-ply wood product according to claim 7 wherein foamers are usedas an additive.
 15. A multi-ply wood product according to claim 7wherein heat binding substances are used as an additive.
 16. A multi-plywood product according to claim 1, wherein said prefabricated crudefiber ply blank is fitted upon a wooden ply, so that, afterhot-pressing, a surface layer of said multi-ply wood product is formed.17. A hot-pressed multi-ply wood product, having at least one woodenply, comprising at least one layer formed by a prefabricated crude fiberply blank including fiber haphazardly oriented in three dimensions, abonding agent, additives, and filler material, said ply blank, by virtueof partial breakdown and reorientation of said fiber, fillsirregularities of the surface of an adjacent wood ply duringhot-pressing, said hotpressing being carried out in a single step, sothat said layer becomes fully bonded to said adjacent ply, wherein saidfiber is ceramic fiber.